Medicament delivery devices are routinely used by persons without formal medical training, i.e. patients where self-management of their condition is increasingly common. These circumstances set a number of requirements for medicament delivery devices of this kind. The injector should be robust in construction, yet easy to use in terms of its operation by a user and the manipulation of the parts. In the case of those with diabetes, many users will be of impaired vision and may also be physically infirm. Devices that are too large of cumbersome may therefore prove difficult to use, particularly someone with reduced dexterity.
Patent Specification U.S. Pat. No. 6,340,357 describes a drug delivery system in which the dose setting is read into an electronic circuit and the dose setting movement of the dose setting elements relative to each other is performed by an electromechanical device, e.g. a motor controlled by the electronic circuit in accordance with the read in dose setting. The electronic control enables the apparatus to intervene by resetting a dose if a miss-handling of the device by the user is detected during dose setting, such as opening of the cartridge holder.
Patent Specification WO 2007/094833 describes a metering system for automatically adjusting for differential thermal expansion/contraction for the efficient, accurate and reproducible metered delivery of fluids. The system allows the metering system drive to re-zero itself to produce an accurate volumetric delivery of fluid from the dispensing container.
It is also known to detect a stall of the motor that drives the dose delivery and to warn the user if a dose fails to be delivered. However, there remains a problem in the resetting of the device following detection of a motor stall event.
It is an aim of the present invention to provide a medication delivery device that alleviates this problem.
According to the present invention, there is provided an injection device for delivering a medicament to a patient, wherein the injector device comprises: a housing; a piston rod for driving a bung of a medicament container; a drive mechanism including a motor for providing an output drive to the piston rod for delivering the medicament; and control means for controlling operation of the device; characterised in that:
the control means comprises: a drive signal generator for generating an input drive signal for the motor; an encoder for generating an encoder output signal indicative of the output drive of the motor; and means for varying the operational control of the device in dependence on a comparison between the input drive signal and the encoder output.
The input drive signal may be stepper pulses for driving the motor. The encoder output signal may be a pulsed signal having a timing characteristic that corresponds to the output drive of the motor. A plurality of reference points may be included in the device, each reference point being indicative of a different operational aspect of the device, including any one or more of: backstop position; dose delivered; door position; drive position; and reset threshold. The reference points preferably relate to the input drive signal such that respective reference points correspond to respective counts of the stepper pulses with reference to a device datum. A comparison between counts of the pulsed encoder output with counts of the stepper pulses may be indicative of motor slip. In the event that motor slip or stall is detected the control means is operative for determining the quantum of slip relative to one of the reference points whereupon said varying means adjusts the operational control of the device according to a predetermined criteria. The variation in operation control may be such as to urge the device to a target operational state. For example, when the quantum exceeds a predetermined threshold value, the motor may be deliberately stalled against a predetermined reference point representative of the target state. For example, the motor drive may be varied so the state of the device is changed to a dose reset position or a ‘cartridge door open state’ for enabling replacement of the cartridge. Alternatively, the motor may be controlled such as to rewind the piston rod to a backstop which defines a device datum or device reset position.
Embodiments of the present invention are advantageous in that the injector automatically initiates a reset action when necessary and without needing user interaction. This leads to an improvement in battery life and an avoidance or reduction in motor stall noise.